University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2019
Views:

422

Pages:

41
Author:

omiliar74be

Chapter 46 and Chapter 7 Fertilization Fertilization in Sea Urchin Fertilization includes several steps: • The acrosomal reaction. • The cortical reaction. • Egg activation. The acrosomal reaction The acrosomal reaction The acrosomal reaction The acrosomal reaction The depolarization of the egg plasma membrane acts as the fast block to polyspermy by preventing additional sperm from fusing with the egg’s plasma membrane. The acrosomal reaction The depolarization of the egg plasma membrane acts as the fast block to polyspermy by preventing additional sperm from fusing with the egg’s plasma membrane. Cortical reaction The formation of fertilization envelope and the removal of the sperm-binding receptors act as a slow block to polyspermy. Cortical reaction Researchers hypothesized that an increase in cytosolic Ca2+ levels triggers cortical granules fusion that leads to the formation of fertilization envelope. To test this hypothesis, they tracked the release of free Ca2+ in Sea Urchin eggs after sperm binding to see if Ca2+ release correlated with the formation of fertilization envelope. Cortical reaction Conclusion: The researchers concluded that Ca2+ release is correlated with the cortical reaction and the formation of fertilization envelope. Egg activation ➢ Fertilization initiates and speeds up metabolic reactions such as cellular respiration and protein synthesis. ➢ These metabolic reactions trigger the onset of embryonic development activating the egg. How does fertilization initiates the metabolic steps that activate the egg? The fertilization increases the Ca2+ release and the rise in Ca2+ concentration that causes the cortical reaction also causes egg activation. Egg activation ➢ In sea urchins, the sperm and egg nuclei fuse after about 20 min from the entry of sperm nucleus into the egg. ➢ The first cell division, which occurs after about 90 min from nuclei fusion, marks the end of the fertilization stage. ➢ Fertilization in other species shares many features with sea urchins, however, there are differences such as the stage of meiosis the egg has reached by the time it is fertilized. Example: the difference between sea urchin egg and human egg. Internal fertilization in mammals It is difficult to study any interactions between the mammalian sperm and egg that take place prior to these gametes making contact because: • The mammalian fertilization occurs inside the oviduct of the female. Internal fertilization in mammals It is difficult to study any interactions between the mammalian sperm and egg that take place prior to these gametes making contact because: • The mammalian fertilization occurs inside the oviduct of the female. • The sperm population ejaculated into the female is probably heterogeneous, containing spermatozoa at different stages of maturation. Internal fertilization in mammals It is difficult to study any interactions between the mammalian sperm and egg that take place prior to these gametes making contact because: • The mammalian fertilization occurs inside the oviduct of the female. • The sperm population ejaculated into the female is probably heterogeneous, containing spermatozoa at different stages of maturation. • There might be multiple mechanisms by which the mammalian sperm can undergo the acrosome reaction and bind to the zona pellucida. Getting the gametes into the oviduct: Translocation and capacitation Translocation • The meeting between the male and female gametes occurs in the ampulla and is facilitated by the female reproductive tract. Getting the gametes into the oviduct: Translocation and capacitation Translocation • In mammalian egg, the extracellular envelope is a separate, thick matrix called the zona pellucida. • The mammalian egg is also surrounded by a layer of cells called the cumulus, which is made up of the ovarian follicular cells that were nurturing the egg at the time of its release from the ovary. • The inner most layer of cumulus cells, immediately adjacent to the zona pellucida, is called the corona radiata. Getting the gametes into the oviduct: Translocation and capacitation Translocation Translocation of the sperm from the vagina to the oviduct involves several processes that work at different times and places including: • Sperm motility. • Uterine muscle contractions. • Sperm rheotaxis. The sperm display rheotaxis that is they migrate against the direction of the flow of liquid from the oviduct to the uterus. Getting the gametes into the oviduct: Translocation and capacitation capacitation • Newly ejaculated mammalian sperm are immature and cannot fertilize the egg; they are unable to undergo the acrosomal reaction or to sense the cues that will eventually guide them to the egg. • To achieve such competence, the sperm must undergo capacitation. • Capacitation: the set of physiological changes by which mammalian sperm become capable of fertilizing an egg. • The physiological changes are accomplished only after a sperm has resided for some time in the female reproductive tract. • As the sperm reach the ampulla, they acquire the ability to fertilize the egg because they have undergone capacitation. Getting the gametes into the oviduct: Translocation and capacitation Capacitation • The molecular processes of capacitation enable the sperm to become hyperactive and prepare the sperm for the acrosome reaction. Getting the gametes into the oviduct: Translocation and capacitation Capacitation • The cholesterol efflux from sperm membrane is thought to change the location of the lipid rafts: isolated regions that often contain receptor proteins that participate in the acrosome reaction and can bind the zona pellucida. • SACY: Adenylate kinase. • Once they are phosphorylated, some proteins migrate to the sperm head. • One of these proteins is Izumo which is critical in sperm-egg fusion. Getting the gametes into the oviduct: Translocation and capacitation Capacitation • The uncapacitated sperm bind actively to the membrane of the epithelial cells in the narrow passage (isthmus), before entering the ampulla of the oviduct. • This binding is temporary and appears to be broken when the sperm become capacitated. In the vicinity of the oocyte: Hyperactivation, thermotaxis, and chemotaxis • Toward the end of capacitation, sperm become hyperactivated: they swim at higher velocities and generate greater force. • Hyperactivation appears to be mediated by the opening of a sperm-specific Ca2+ channel in the sperm tail. • Hyperactivation of the sperm may enable them to respond differently to the fluid current. • Uncapacitated sperm move in a planar direction, while capacitated ones rotate around their axis. In the vicinity of the oocyte: Hyperactivation, thermotaxis, and chemotaxis What does provide the sperm with direction? • Heat • Molecules secreted from the oocyte and cumulus cells • Thermotaxis: is the ability to sense temperature differences and preferentially swim from cooler to warmer sites. It is found in capacitated sperm. • Chemotaxis: is the movement of a cell down a chemical gradient such as the movement of a capacitated sperm toward the egg during the last stages of sperm migration following molecules secreted from the oocyte and its accompanying cumulus cells. In the vicinity of the oocyte: Hyperactivation, thermotaxis, and chemotaxis • In humans, progesterone (a chemotactic compound) has been shown to bind to a receptor that activates Ca2+ channels in the cell membrane of the sperm, leading to sperm hyperactivation. • This activation of Ca2+ channels by progesterone takes place only when the intracellular pH of the sperm is increased by the influx of HCO3- ions into the cell. Acrosome reaction and recognition at the zona pellucida • Zona pellucida in mammals plays a role similar to that of the vitelline envelope in invertebrates. • However, it is thicker and more dense structure than the vitelline envelope. • The human zona pellucida has four major glycoproteins: ZP1, ZP2, ZP3, and ZP4. Mouse zona pellucida Acrosome reaction and recognition at the zona pellucida • The binding of sperm to the zona pellucida is relatively species-specific and a species may use multiple mechanisms to achieve this binding. • In mouse, the acrosome reaction occurs prior to the sperm binding to the zona. Acrosome reaction and recognition at the zona pellucida • A successful sperm that can actually fertilize the egg, is the one that had undergone the acrosome reaction by the time it was first seen in the cumulus before reaching the surface of zona pellucida. • On the other hand, acrosome intact sperm is almost always unsuccessful sperm that remains bound to the zona pellucida without undergoing the acrosome reaction or progressing to the egg cell membrane. Acrosome reaction and recognition at the zona pellucida • In gain of function experiments, ZP2 was shown to be critical for human sperm- egg binding. • Transgenic mouse oocytes were constructed that expressed the three normal mouse zona proteins and also one of the four human zona proteins (human ZP1, ZP2, ZP3, and ZP4) Acrosome reaction and recognition at the zona pellucida • The binding of sperm to zona pellucida is relatively species-specific and a species may use multiple mechanisms to achieve this binding. • In mice, there is evidence that acrosome-intact sperm can bind to ZP3, and that ZP3 can cause the acrosome reaction directly on the zona. • There may be several mechanisms to initiate the acrosome reaction and to bind to and penetrate the zona pellucida. • These mechanisms may act simultaneously, or perhaps one mechanism is used for a acrosome-intact sperm and another for acrosome reacted sperm. Gamete fusion and prevention of polyspermy • In mammals, it is not the tip of the sperm head membrane that makes contact with the egg membrane, but it is the side of the sperm head membrane. • The junction between the inner acrosomal membrane and the sperm cell membrane is called the equatorial region, and this is where membrane fusion between sperm and egg begins Gamete fusion and prevention of polyspermy • The mechanism of mammalian gamete fusion involves Izumo protein on sperm side and Juno/CD9 proteins on the egg microvilli. Gamete fusion and prevention of polyspermy • In mammals, only slow block to polyspermy occurs and the electrical fast block to polyspermy has not been detected. • The slow block to polyspermy occurs when the enzymes released by the cortical granules modify the zona pellucida receptor proteins such that they can no longer bind sperm. • ZP2 is clipped by the protease ovastatin and loses its ability to bind sperm. Gamete fusion and prevention of polyspermy • Another slow block to polyspermy involves the Juno protein. • As the sperm and egg membranes fuse, the Juno protein appears to be released from the plasma membrane. • Also, this soluble Juno protein can bind sperm in the perivitelline space between the zona pellucida and oocyte. Fusion of genetic material • In mammals, the sperm enters the oocyte while the oocyte nucleus is arrested at metaphase II. • The oscillation of Ca2+ level brought about the sperm entry activates a kinase that leads to the proteolysis of cyclin and securin. • Eventually, these hydrolysis events result in the formation of a haploid female pronucleus. SMC protein: Structural maintenance of chromosome protein Kleisin Fusion of genetic material • DNA synthesis (DNA replication) occurs separately in the male and female pronuclei. • The centrosome associated with the male pronucleus generates aster. • The centrosome split into two to organize bipolar microtubule array. Asters Male pronucleus Female pronucleus Bipolar microtubule array Fusion of genetic material • Upon meeting, the two nuclear envelopes break down. • The chromatin from male and female pronuclei condenses into chromosomes that intermix on the metaphase equator. • The mitotic spindle initiates the first mitotic division. Male pronucleus Female pronucleus Pronuclei fusion Alignment of intermixed chromosomes Fusion of genetic material • Each sperm brings into the egg, in addition to its nucleus, its mitochondria, centrioles (centrosome), and a tiny amount of cytoplasm. • The sperm cytoplasm contains enzymes that activate egg metabolism, as well as RNA fragments that may alter gene expression. Activation of mammalian egg • As in other animals, the transient rise in cytoplasmic Ca2+ level is necessary for egg activation in mammals. • The sperm entry induces a series of Ca2+ waves that last for hours and terminate in egg activation and formation of male and female pronuclei. • Egg activation in mammals involves the resumption of meiosis, cortical granule exocytosis and release of the inhibition on maternal mRNA. Activation of mammalian egg • It appears that an activator of Ca2+ release in mammals is a soluble sperm phospholipase C (PLC) enzyme known as PLCζ (zeta) PLCζ Phosphatidylinositol 1,4,5- bisphosphate (PIP2) Inositol 1,4,5-triphosphate (IP3) + diacylglycerol (DAG) Activation of mammalian egg • In mammals, the Ca2+ released by IP3 binds to a series of proteins including: • Calmodulin-activated protein kinase which is important in eliminating the inhibitors of mRNA translation. • MAP kinase which allows resumption of meiosis. • Synaptotagmin which helps initiate cortical granule fusion.

Chapter 46 and Chapter 7 Fertilization

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